In the aircraft industry a large number of rivets are utilized when fabricating a single aircraft. Because of the large number of rivets which are used, and also because of the requirements for virtually indefinite life of the rivets, much attention has been given in the industry to various methods and apparatus for riveting. One riveting method and apparatus which has been utilized by the industry for a number of years is shown in U.S. Pat. No. 3,557,442. This patent discloses the utilization of slug rivets to secure two workpieces together, the workpieces initially being clamped together. This patent teaches that the upper rivet forming anvil is initially extended to a full down locked position, with all of the rivet upsetting force then being applied by the upward movement of the lower rivet forming anvil, the ends of the rivet being simultaneously formed. During the riveting process the surface of the workpieces will move relative to a fixed work plane. This is referred to in the industry as a "wink".
The process of the foregoing patent requires that the workpieces be initially stacked together, held in a fixture, and then subsequently clamped together prior to riveting. When the workpieces are rigidly held externally relative to the apparatus, such as in a rigid fixture, and the clamps are brought to the opposite sides of the workpieces it is desirable that the clamps not apply any bias force so as to avoid any deformation of the workpieces. Related to the requirement of avoiding any deformation of the workpieces is the need to establish a work line reference for the automatic fastening machine. In particular, there is need to coordinate operation of the fastening machine with variations in the work line. This is essential in spar work where there may be no common work plane. In other structures, such as a rigidly held wing panel, it is also needed due to random variations which the machine needs to accommodate.
In situations where the workpieces are not rigidly held, and are therefore allowed some degree of movement when clamped, prior art clamps can apply a bias force to the workpiece and thus wink or move the workpieces during clamping.
As the workpieces are winked or moved during the clamping or during the squeeze cycle they will have a tendency to oscillate before returning to their original position. This oscillation could delay the next operation. Additionally, if a slug rivet can be formed without winking better control of the position of the slug can be achieved. As there would be no movement of the workpiece even greater uniformity of the bulging of the rivet may be achieved which is desirable for rivet fatigue life cycles. In addition, by not moving the workpiece during riveting there is a potential for even faster rate times.
It would, therefore, be desirable to provide a new and improved apparatus for clamping and fastening workpieces which sense contact with the workpiece surface to establish a reference work plane and which applies no bias force to the workpieces so as to avoid deformation thereof.